Diesel engines (used for example in vehicles, ships, and machines) using light oil as a fuel are fuel-efficient, but there are recently some problems because the exhaust gas has adverse effects on the environment. Accordingly, biodiesel fuels are attractive as an alternate fuel for use in the diesel engines.
The biodiesel fuel is a fuel containing fatty acid alkyl esters obtained, for example, by ester exchange of fatty acid triglycerides (referred to also as “fatty acid glycerides”), principal components of the vegetable oils/fats, animal oils/fats or the waste oils/fats thereof, with a lower alcohol or by esterification of the free fatty acid products generated by hydrolysis of the fatty acid triglycerides with a lower alcohol.
Carbon dioxide generated from the biodiesel fuel is the carbon dioxide originally presence in air that was absorbed by vegetables for oil/fat raw materials such as soybean and rapeseed. Therefore, the biodiesel fuel, which is a carbon-neutral fuel that does not destroy the carbon balance on earth, is attractive as an alternative fuel for fossil fuels.
Alkali-catalyzed methods have been used as an industrial production method for production of the biodiesel fuel fatty acid alkyl esters. The alkali-catalyzed method is a method of ester-exchanging the fatty acid triglycerides with an alcohol in the presence of an alkali metal catalyst at a temperature close to the boiling point or at normal temperature. The method, which does not demand such a high-temperature and high-pressure condition as those by the supercritical methanolysis method and has a catalytic activity in the ester-exchange reaction higher than that for example by acid-catalyzed methods, allows relatively more cost-effective and easier progress of the ester-exchange reaction. Therefore, the alkali-catalyzed method is important as an industrial production process for the biodiesel fuel.
However, the alkali-catalyzed method also has various disadvantages. First, the biodiesel fuel for use should be high-quality fatty acid alkyl esters extremely lower in impurity content. In particular, disadvantages of a biodiesel fuel by the alkali-catalyzed method include by-production of an alkali soap, difficulty of phase separation between the fatty acid alkyl ester phase and the glycerol phase because of the alkali soap, and difficulty in separating activated clay added for neutralization of the reaction solution and the metal alcoholate used as an alkali metal catalyst.
In addition, ester-hydrolyzing impurities such as water are contained in greater amounts for example in the waste oil/fat raw materials, and it is important to remove water in the oil/fat raw material prior to reaction. Patent Document 1 discloses a method of producing fatty acid lower alcohol esters having an adjusting step of adding water to an esterification mixture and agitating the mixture, as the method of overcoming such problems associated with separation.
Patent Document 1: Japanese Unexamined Patent Publication No. Sho56-65097